Photonic devices for optical network management and wavelength multiplexing and demultiplexing applications have been extensively researched for a number of years. A significant class of such devices is commonly called "planar lightwave circuits" or "planar lightwave chips" or just PLC's. PLC's comprise technologies wherein complex optical components and networks are disposed monolithically within a stack or stacks of optical thin films supported by a common mechanical substrate such as a semiconductor or glass wafer. PLC's are designed to provide specific transport or routing functions for use within fiberoptic communications networks. These networks are distributed over a multitude of geographically dispersed terminations and commonly include transport between terminations via single-mode optical fiber. For a device in such a network to provide transparent management of the optical signals it must maintain the single-mode nature of the optical signal. As such, the PLC's are commonly, though not strictly, based on configurations of single-mode waveguides. Since optical signals do not require return paths, these waveguide configurations do not typically conform to the classic definition of "circuits", but due to their physical and functional resemblance to electronic circuits, the waveguide systems are also often referred to as circuits.
A key performance issue in the practical application of PLC's is the efficiency of the circuit in transporting the optical energy of the signal. This performance is characterized in terms of the fraction of energy lost from the signal passing through the device, expressed as "loss" in units of decibels (dB) or "loss rate" in units of dB/cm. The standard family of materials for PLC waveguides, widely demonstrated to have superior loss characteristics, is based on silicon dioxide (SiO.sub.2), commonly called silica. The silica stack includes layers that may be pure silica as well as layers that may be doped with other elements such as Boron, Phosphorous, Germanium, or other elements or materials. The doping is done to control index-of-refraction and other necessary physical properties of the layers. Silica, including doped silica, as well as a few less commonly used oxides of other elements, are commonly also referred to collectively as just "oxides". Furthermore, although technically the term "glass" refers to a state of matter that can be achieved by a broad spectrum of materials, it is common for "glass" to be taken to mean a clear, non crystalline material, typically SiO.sub.2 based. It is therefore also common to hear of oxide PLC's being referred to as "glass" waveguides. Subsequently, the moniker "silica" is used to refer to those silicon oxide materials suitable for making PLC waveguides or other integrated photonic devices.
One of the promising features of PLC's is the ability to integrate transport and interconnect functions with dynamically selectable functions such as routing, switching, attenuation, and programmable filtering. These dynamic functions typically are used to provide integrated, solid-state replacements for functions that would otherwise need to be performed by discrete components using mechanical-displacement-based devices. These functions are achieved based on the phenomena that certain stimuli applied to the active area of a waveguide device will change the magnitude of the refractive indices in and around the waveguides in that area. Typically-used stimuli for changing the refractive index are electric field (electro-optic), heat (thermo-optic), or dynamic stress (acousto-optic). Less commonly, other effects such as piezo-optic, static-stress, photo-refractive, etc., are employed. in waveguide applications In the current state-of-the-art, thermo-optics is being accepted for the broadest range of applications and can provide a more predictable response to the randomly varying polarization of the optical signal presented by the telecommunications network. As such, discussions in this application will focus on thermo-optics, where a heat source in the vicinity of the active region of the waveguide device is used to change the temperature and thus select an index change and effect the operation of the device. The optical behavior of these devices is simply determined by the refractive-index distributions generated. It should be recognized that it would be readily apparent to those skilled and experienced in these technologies that the devices and structures described in this teaching can be applied in substance to electro-optic and other methods of stimulating the appropriate refractive-index profiles.
As previously stated, silica waveguides have superior loss characteristics for transporting the optical signals used in these communications networks. Their performance is also very stable with respect to reasonable changes of their local environment. This behavior is desirable for transport and interconnect functions, but it leads to difficulties in achieving any of the stimulated functions. There are a few thermo-optic devices for these applications currently being made from silica waveguides. Since the index changes that can be commanded are on the order of only 10.sup.-4 per micron and can not be localized within regions a few microns wide, thermo-optic silica devices exhibit marginally acceptable performance while requiring uncommonly sophisticated control of the driving conditions.
Other classes of materials, notably optical polymers, have superior response for stimulated functions. They however can not match the low-loss transport qualities of good silica at relevant optical wavelengths. There also are various thermo-optic devices and circuits being made with optical-polymer waveguides and they achieve reasonable and robust levels of performance for their active functions, but the overall device loss is fairly high, typically a few dB for simple functions like a 2.times.2 switch. Currently, the main impediment to wide-spread deployment of such solid-state switches in fiber-optic telecommunications networks is a lack of availability of robust switches having less than 1 dB of device loss.
When one wishes to make a thermo-optic waveguide switch, it would be expected that the optical signals should be appropriately coupled with high efficiency between the input and output ports in response to the driven stimulus, and that this performance should not be significantly altered by any undriven stimulus. That is, for a thermo-optic switch one would want to simultaneously achieve: (1) the heating signal applied or removed by direction of the user should robustly establish the switching; and (2) any heat applied or removed by conditions arising from the environment should not effect the switching. To exhibit this behavior the device must be shielded from environmental changes and/or the switch must be designed to be sensitive to an applied heating that cannot occur naturally and insensitive to thermal patterns that can occur naturally. The former approach is occasionally used when there is no other way to achieve superior performance, but the latter approach is invariably preferable when available, since stabilization techniques are more expensive to realize and support and typically reduce the reliability of the overall device.
There is a class of devices, commonly referred to as "digital optical switches" that exhibit some very desirable characteristics in regard to the conditions cited above. Firstly, the response of these devices saturates as the driving temperature is increased, so a device may be "over driven" to isolate the response from any changes not arising from the drive signal. Secondly, these devices are typically configured to respond only to a strong thermal index gradient of order 10.sup.-4 per micron or greater. Such gradients are only achieved with highly-localized heating such as can be done with a photolithographically-patterned resistive heater, but can not arise from naturally occurring thermal phenomena or noise. Thirdly, devices of these types can typically be designed with a complimentary symmetry such that activating a heater in one position on the device can saturate the device to one state, while activating a heater at a different position on the device can saturate the response to the complimentary state. This provides even further noise immunity. The archetypal digital-optical switch is the adiabatic Y-Junction (Burns, US04070092, 1978). An existing basic Y-junction switch is depicted in FIG. 1. Passing an appropriate current through either heater will impose a thermal gradient across the junction region and down to the substrate with the higher temperature side of the gradient of course being on the side of the driven heater.
If this device were made in optical polymer, which exhibits a decrease in index with increasing temperature, strong optical coupling will be established between the primary waveguide 101 and the waveguide arm disposed opposite the driven heater. That is, driving heater 111 will select the optical path between waveguides 101 and 104, and conversely, driving heater 112 will select the optical path between waveguides 101 and 103. However, an all-polymer digital optical switch suffers from high photonic loss. Consequently, the use of all-polymer digital optical switches can require a number of optical amplifiers to be included within the optical circuits.
It is not practical to make a digital optical switch in silica. Silica exhibits little change in refractive index with temperature and has a moderately high thermal conductivity. It is consequently very difficult to provide sufficient heat to raise the temperature of the silica sufficiently that switching occurs.
Another configuration of the digital-optical switch is the parabolic directional coupler depicted in FIG. 2 (Syahriar et al., 1998). Here, when the heater is undriven, the thermal properties of the substrate ensure that there is no thermal gradient across the junction, hence no index gradient. The device is stable in the "bar" state where waveguide 201 is strongly coupled with waveguide 204 and waveguide 202 is strongly coupled to waveguide 203. When heater 211 is driven, an index gradient across the coupling region is established and the device saturates to the "cross" state with waveguide 201 strongly coupled to waveguide 203 and waveguide 202 strongly coupled to waveguide 204.
As compared to other materials for waveguides, silica has a rather low thermo-optic coefficient around 2.times.10.sup.-5 per degree C. where the thermo-optic coefficient is defined as the change in index per incremental change in temperature, typically measured in units of (degree C.).sup.-1. Silica also has a moderately high thermal conductivity around 15.degree. C. per Watt-Meter. A large temperature increase is needed to change the refractive index, yet much of the heat applied to the silica is rapidly conveyed away by the silica. It is consequently not practical in silica to create the index gradients needed for digital-optical switch structures. Similar issues are encountered with waveguides made in semiconductor material systems, where the thermo-optic response is an order of magnitude greater, but so is the thermal conductivity, and the steep index gradients needed for digital-optical switches are equally or more difficult to establish with simple heaters. When strong index gradients are not achievable, devices must typically rely on interference. The optical signal is split into two or more optical signals travelling along separate optical paths. The paths must diverge to tens of microns or more separation so thermo-optic refractive-index changes may be applied to one path independently of the other. The signals must then be recombined, and how they are routed through the recombination region will be determined by the relative phases accumulated by the optical signals in each separate path. The resulting device, typically a "Mach-Zehnder"--type interferometer, is depicted in FIG. 3. The response of an interferometer is cyclic and does not saturate with increasing stimulus. They are however very sensitive and can produce a full-scale output swing from an index change of about 5.times.10.sup.-5 along a 1-cm optical path. Interferometers require extremely precise control of the relative phase accumulation in the optical paths, but they can be especially useful when live feedback is available and acceptable for active control of the drive signal.
In contrast to the inorganic waveguide materials, many optical polymers have a magnitude of the thermo-optic response that is 10-20 times greater (or more) than silica and their thermal conductivity is around 1/10.sup.th that of silica. Much steeper thermal index gradients can be achieved in optical polymers and digital-optical-switch devices can easily be realized. However, waveguides made in polymers have significantly higher photonic loss rates than good silica waveguides. Consequently, as a rule optical polymers are excellent materials for making small, active photonic structures, but introduce performance penalties in loss when also used as the photonic interconnect between devices and to provide the chip's optical I/O terminations. Conversely, silica an ideal interconnect medium on photonic circuits but makes for poor active devices.